1. Technical Field
The present invention relates to a piezoelectric device for an ultrasonic sensor.
2. Related Art
In the related art, an ultrasonic sensor using electromechanical transformation characteristics of a piezoelectric element is provided. In the ultrasonic sensor, an ultrasonic wave (transmitting ultrasonic wave) is transmitted by driving the piezoelectric element by supplying electric signals to the piezoelectric element. In addition, the piezoelectric element receives the ultrasonic wave (reflected ultrasonic wave) which is reflected from a measurement object, thereby driving the piezoelectric element and obtaining the electric signals. In the ultrasonic device equipped with the ultrasonic sensor, information (positions, shapes, or the like) relating to the measuring object is detected based on these electric signals, that is, waveform signals of the transmitting ultrasonic wave or the reflected ultrasonic wave.
This type of the ultrasonic sensor is classified into a transmit-only type ultrasonic sensor optimized for transmitting the ultrasonic wave, a receive-only type ultrasonic sensor optimized for receiving the ultrasonic wave, and a transmission-reception integral-type ultrasonic sensor optimized for both transmitting and receiving the ultrasonic wave. In addition, the ultrasonic sensor is also classified into a type of the ultrasonic sensor in which the piezoelectric element side of a vibrating plate is set to a passing area of the ultrasonic wave (so-called ACT surface type), a type of the ultrasonic sensor in which a side opposite to the piezoelectric element of the vibrating plate is set to the passing area of the ultrasonic wave (so-called CAV surface type), or the like.
Here, in a case where improvement in the receiving properties of the ultrasonic sensor is obtained, from the viewpoint of obtaining high deformation efficiency and excellent ferroelectricity, an increase in the residual amount of polarization can be obtained by suppressing an initial deflection to the CAV surface side of the vibrating plate. That is, it is preferable that the initial deflection of the vibrating plate is set to the ACT surface side instead of the CAV surface side by tensile stress. For example, in JP-A-2013-175879, a CAV surface type ultrasonic sensor in which iridium (Ir) is used for an upper electrode is disclosed. The Ir has characteristics of great tensile stress. By using the characteristics, the initial deflection of the vibrating plate can be set to the ACT surface side instead of the CAV surface side by the tensile stress. However, if the Ir is used as an electrode material, there is a possibility that the residual amount of polarization of the piezoelectric element is reduced, and the receiving properties of the ultrasonic sensor are deteriorated.
It can be also considered that platinum (Pt) which is used as the electrode material in common with the Ir and which can greatly maintain the residual amount of polarization of the piezoelectric element is applied. However, the tensile stress of the Pt is lower than that of the Ir, and the initial deflection to the CAV surface side of the vibrating plate becomes greater. Specifically, the piezoelectric element including a piezoelectric layer which is formed by a chemical solution deposition (CSD) method especially has such tendency.
Accordingly, in a case where the Ir is applied as the electrode material of the ultrasonic sensor, it can be expected that the deformation efficiency is improved by the tensile stress of the Ir. However, since the residual amount of polarization is reduced than a case where the Pt is applied as the electrode material of the ultrasonic sensor, there is a possibility that the receiving properties are deteriorated as a result. On the other hand, in a case where the Pt is applied as the electrode material, since the tensile stress of the Pt is smaller than that of the Ir, it is difficult to obtain high receiving properties.